A bipartisan group of U.S. senators will introduce Tuesday a bill that would end the federal ban on medical marijuana, reversing a decades-old law that prohibits even military doctors from prescribing cannabis to war veterans suffering from post-traumatic stress.

The legislation, the first of its kind, would "allow patients, doctors and businesses in states that have already passed medical marijuana laws to participate in those programs without fear of federal prosecution," Sens. Rand Paul (R-Ky.), Kirsten Gillibrand (D-N.Y.) and Cory Booker (D-N.J.) said in a statement Monday, according to the Washington Post.

Paul has been an outspoken critic of American drug policy for years. Now the likely candidate for president is preparing to bring his marijuana reform argument to the Republican Party's 2016 nominating contest. For Republicans seeking a return to the White House after President Barack Obama's consecutive triumphs, that should be welcome news.

"It has such a nice natural connection to a lot of Republican constituencies," John Hudak, a fellow at the Brookings Institution and managing editor of its FixGovblog, told Mic. "You can frame marijuana legalization around free market ideas, issues of liberty, issues of federalism and states' rights — all really nice topics and talking points for Republican candidates. A lot of issues touch on maybe one of those or two of those, but marijuana really touches on quite a few."

Hudak said Republicans, who have mostly scaled back their opposition to legalization in the states, "are doing something they are usually terrible at — looking down the road at the demographic landscape and adjusting their policies accordingly."

"They're not doing it on same-sex marriage effectively, they're not doing it on health care effectively, they're not doing it on a lot of social issues effectively," he said, "but it looks like they're starting to see the light on legalization."

Better late than never. In 2016, young voters who overwhelming support marijuana legalization are expected to return to the polls after their traditional midterm no-show in 2014.

"They're sort of self-reinforcing forces in the electorate that create a really positive recipe to assist legalization advocates," Hudak said."It's a hot issue. For a candidate, you talk about it and it makes waves."

Some of those early Republican frontrunners are trying to ride that wave, while others are planning to fight a tide of increasing public support. From most supportive to least, here's where six of the GOP's likely 2016 presidential candidates stand on the future of marijuana in America:

1. Sen. Rand Paul (R-Ky.)

Does he regret it? "Let's just say I wasn't a choir boy when I was in college and that I can recognize that kids make mistakes, and I can say that I made mistakes when I was a kid," Paul told WHAS-TV in December 2014.

True believer: When it comes to policy, Paul is the undisputed king of the GOP's 2016 crop. His commitment to scaling back marijuana prohibition is deeply rooted in the libertarian politics that made his father, former Rep. Ron Paul (R-Texas), a cult figure on the fringes of the Republican Party. The son, though, is committed to winning.

"Paul's someone who is as close to pro-legalization as a Republican is going to get anytime soon," Hudak said. "And he does so for a variety of reasons: liberty issues, states rights issues and economic issues. I think he understands that the drug war in this country has failed. This seems like a very sincere reflection of his policy ideas."

Paul's primary focus seems to be something closer to decriminalization. Much of his rhetoric centers on concerns about mandatory minimum sentencing, which can lead to long prison sentences for nonviolent drug offenses and disproportionately injures minority communities.

"If your kid was caught selling marijuana or growing enough that it's a felony conviction, they could be in jail for an extended period of time, they also lose their ability to be employable," Paul told WHAS-TV in November 2014. "So I want to change all of that. I want to lessen the criminal penalties on it."

In addition to their joint effort to end the federal ban on medical marijuana, Paul and Booker have promised to work together on sentencing reform. A formal proposal could come later this year, sometime after Paul makes official his expected run for president. Drug and prison policy are expected to be at the core of his campaign appeal to young voters.

2. Sen. Ted Cruz (Texas)

Regret it? "When he was a teenager, he foolishly experimented with marijuana. It was a mistake, and he's never tried it since," a representative told the Daily Mail earlier this month.

For the states: Cruz made headlines during the recent Conservative Political Action Conference, where he was asked about laws legalizing recreational marijuana use in Colorado and Washington state.

"I actually think this is a great embodiment of what Supreme Court Justice Louis Brandeis called 'the laboratories of democracy,'" Cruz said. "If the citizens of Colorado decide they want to go down that road, that's their prerogative. I don't agree with it, but that's their right."

Not everyone agrees. State and federal law is in tension today in Colorado and Washington state. Had the Obama administration not decided to allow implementation of those state laws, there would have been no responsible way for either state to enact voter-approved legalization. A new president with a dimmer view of marijuana policy could instruct the Drug Enforcement Agency to resume prosecuting anyone who violates federal law, which still classifies weed as a Schedule I controlled substance.

Which is not to say Cruz has always been crystal clear about his feelings. In January 2014, he seemed to challenge President Barack Obama's decision not sic the feds on Colorado.

"You can go to Congress, you can get a conversation, you could get Democrats and Republicans who would say, 'We ought to change our drug policy in some way,' and you could have a real conversation, you could have hearings, you could look at the problem, you could discuss commonsense changes that maybe should happen or shouldn't happen," Cruz said in speech to the Texas Public Policy Foundation in January 2014.

"This president didn't do that. He just said, 'The laws say one thing' — and mind you these are criminal laws, these are laws that say, 'If you do X, Y, and Z you will go to prison.' The president announced, 'No, you won't.'"

But critics who read those comments as an attack on legalization missed the point. Cruz wasn't targeting people who bought and sold pot in Colorado. Rather, he was calling out Obama for deciding not to enforce federal law without consulting Congress. It's a familiar political point, but not one that supersedes his longstanding support for states' rights.

3. Wisconsin Gov. Scott Walker

Ever get high?No. "The wildest thing I did in college was have a beer," Walker said at a news conference in February.

So you're saying there's a chance? The union-breaking, tax-cutting Walker has emerged as one of the favorites in the early stages of the Republican presidential nominating process. Conservatives celebrate Walker for his brash right-wing economic policy, but his pot politics are much fuzzier.

To start, he took the small but important step of signing a bill to legalize cannabidiol, or CBD, an anti-seizure drug that contains small amounts of THC, the active chemical ingredient in marijuana. THC levels in CBD are so low it's not even considered a medical marijuana product, but it had been banned in the state.

On the broader question of legalization, both for medicinal and recreational use, Walker has resisted without drawing a hard line. When Democratic state lawmakers wrote up a legalization bill, he seemed interested, but not convinced.

"I don't think you're going to see anything serious anytime soon here, but if other states did, maybe in the next legislative session there'd be more talk about it," Walker told WITI-TV earlier this month, also mentioning a recent conversation with Colorado Gov. John Hickenlooper, a Democrat, about the windfall of new tax money that followed legalization in the Rockies.

"It may be something that resonates in the future. I just don't see any movement for it right now."

There is no reason to expect Walker is going to change his mind anytime soon. He still refers to marijuana as a "gateway drug" and told the Milwaukee Journal-Sentinel he thought it was a "big jump between someone having a beer and smoking marijuana," despite increasingly powerful evidence that pot is significantly safer than alcohol.

4. Former Florida Gov. Jeb Bush

Does he regret it? Meh. "It was pretty common," Bush said about his hash-smoking habit in a Boston Globe report on his prep school days. "The first time I really got stoned was in Jeb's room," an old friend told the Globe. "He had a portable stereo with removable speakers. He put on Steppenwolf for me." They listened to "Magic Carpet Ride."

Things have changed: That ride is over now. Bush has become a vocal opponent of legalization for any purpose. With medical marijuana on the ballot in Florida last fall, Bush released a statement urging voters to reject the initiative, warning that the passage would allow "large-scale, marijuana operations to take root across Florida, under the guise of using it for medicinal purposes."

Amendment 2 was defeated on Election Day, falling short of the 60% vote needed for approval.

Bush maintained his opposition during an appearance at CPAC on Feb. 27, saying that while he disagrees with proponents of legalization, he wouldn't interfere with a state's decision.

Hudak isn't convinced. Bush is the proud owner of a deeply conservative political record he burnished in part by courting Florida's Christian right.

"I think anyone who is working hard to connect to the religious right is going to worry legalization supporters," Hudak said. "There's a risk there that values voters, the moral right-wing voters are going to really resist the liberalization of marijuana policy.

"I think for supporters those are the biggest worries: How big of a voice is the religious right going to have and is the religious right going to move candidates who are not firm on the issue to take a more hardline stance."

Expect Bush to face more questions on the particulars of his policy as the campaign makes its way to Florida, which is expected to have another medical marijuana initiative on the ballot in 2016.

5. Sen. Marco Rubio (Florida)

Why not? "If I tell you that I haven't, you won't believe me," he told Fusion in February 2014. "And if I tell you that I did, then kids will look up to me and say, 'Well, I can smoke marijuana because look how he made it.'"

Twisted "Web": Rubio says he approves of medical marijuana, as long as it doesn't get you high. He also says he'd support a medical marijuana bill in Florida, but he didn't back the one on the ballot in 2012.

"If there are medicinal uses of marijuana that don't have the elements that are mind-altering or create the high but do alleviate whatever condition it may be they are trying to alleviate, that is something I would be open to," Rubio told reporters, including one from the Tampa Bay Times, in July 2014.

Rubio would only back opening the medical market to "noneuphoric strains such as 'Charlotte's Web,'" according to the Tampa BayTimes report, which also cites him calling the 2015 initiative a "ruse" that would simply be used to grease the way for full legalization.

And that is something Rubio is dead set against. He is the only likely candidate in this group to explicitly call for federal law to be enforced in Colorado and Washington state, effectively overruling voters there and potentially reinstating prohibition.

"The bottom line is, I believe that adding yet another mind-altering substance to something that's legal is not good for the country," he told ABC News in May 2014. "I understand there are people that have different views on it, but I feel strongly about that."

6. New Jersey Gov. Chris Christie

The hardliner: Of all the likely top-tier GOP presidential candidates, none has been quite so loud and proud about his opposition to marijuana legalization than Christie. But it's not just his blustering style that sets him apart — he rails against any suggestion that pot policy needs fixing and leans on some of the most factually dubious and damaging logic to make his point.

"I believe that this is a gateway drug into other more serious drugs, I think it sends a wrong message to our kids and I don't think it makes anybody a better or more productive person," Christie told a caller on his radio show in March 2014. As all reliable science and peer-reviewed studies have confirmed, the idea that marijuana is a "gateway drug" is untrue.

What about the tax revenue that could follow legalization?

"For the people who are enamored with the idea with the income, the tax revenue from this, go to Colorado and see if you want to live there," Christie said in April 2014. "See if you want to live in a major city in Colorado where there's head shops popping up on every corner and people flying into your airport just to come and get high. To me, it's just not the quality of life we want to have here in the state of New Jersey and there's no tax revenue that's worth that."

Just one problem. As the Huffington Post explained, there is a quite a bit of evidence suggesting that Colorado has a better standard of living than Christie's "Garden State."

Medical marijuana is legal in New Jersey — it was passed in 2009 and signed by Christie's predecessor — but enrollment numbers are extremely low.

Christie says that's because the current program is just a "front for legalization."

Christie's hometown newspapertells a different story. New Jersey's Star-Ledgerquotes from a range of parties that argue low enrollment numbers in the state are not for lack of demand but a chilling effect caused by the governor's policies:

Some lawmakers, dispensary operators, and patients blame the low enrollment on the program's strict rules, high costs, the small amount of doctors willing to recommend patients, and Christie's lack of involvement in enhancing participation.

"There is so much reticence on the administration's part, I don't know how you break that logjam," state Assemblyman Reed Gusciora (D-Mercer), one of the lead sponsors of the law. "All they have to do is open their eyes."

So what would a "President Chris Christie" do about states like Colorado and Washington? He was asked that very question while campaigning for Republicans in New Hampshire last summer.

"Probably not well."

From practical political questions to the broader societal implications, Christie is the most serious threat in the Republican field to the long fight for marijuana policy reform. Luckily for Republicans, they will have a long and loud primary competition to sort out the party's position going into the 2016 general election season. And with a diverse — on weed issues, at least — menu of candidates to choose from, they'll only have themselves to blame if their man goes up in smoke next year.